With the rapid development of social economy, people's demand on information is increasing dramatically. Thanks to a huge potential bandwidth capacity of the optical fiber of approximately 30 THz, the optical fiber communication has become one of the most important technologies that support the increment of the communication traffic.
In the OTN, the technology of mapping and enveloping a client signal for being transported in the OTN is called a digital enveloping technology, which includes, for example, a mapping/multiplexing structure of an optical channel transport unit (OTU), and time division multiplexing and client signal mapping technical means of an optical channel data unit-k (ODUk).
In order to transmit a client signal, first, the client signal is mapped to an optical channel payload unit-j (OPUj), in which j represents a supported bit rate, which may be 1, 2, and 3 respectively representing the rate levels of approximately 2.5 Gbit/S, 10 Gbit/S, and 40 Gbit/S, then an overhead of the OPUj is added, so as to form the OPUj. Next, a channel overhead of an optical channel data unit-j (ODUj) is added to the OPUj, so as to form the ODUj. Then, an OTU overhead and a forward error correction (FEC) overhead are added to the ODUj, so as to form an optical channel transport unit-j (OTUj). Then, the OTUj is loaded to a certain wavelength and transmitted.
When the client signal is mapped to the OPUj, in order to transport different types of client signals, a plurality of service mapping methods is provided in the OTN specification, including a constant bit rate (CBR) signal mapping method, a generic framing procedure (GFP) frame mapping method, and an asynchronous transfer mode (ATM) cell flow mapping method. As the data services are continuously increasing, new demands are proposed on the full rate transparent transport capability of the OTN, so that the CBR mapping manner is applied more and more widely.
In the prior art, a CBR mapping method is provided, and an optical channel payload unit-k (OPUk) frame structure adopted in the method is as shown in FIG. 1. In such a frame structure, a Cbyte indicates a client signal byte number mapped within one OTN frame period, so that a payload area of the OPUk with a byte space of 3808×4=15232 bytes requires totally a length of 14 bits for indication, that is, the Cbyte occupies 2 bytes. The correctness of the Cbyte value directly influences a reliability for transporting the client signals, so that in the prior art, three same Cbyte values are transported in each frame, and errors are filtered by adopting a majority vote manner, thereby improving the reliability for transporting the client signals.
When the client signals are mapped to OTN virtually concatenated OPUk-Xv (X virtually-concatenated OPUks) by adopting the prior art CBR mapping method, virtual concatenation overhead (VCOH) and payload structure identifier (PSI) occupy a space of 4 bytes in each OPUk overhead of OPUk-Xv, so that only 4 bytes in the 16th column are left for the Cbyte. Thus the demands on the space indication and majority vote cannot be satisfied, and the Cbyte only occupying 2 bytes cannot indicate a byte space under a situation of over 5 virtually-concatenated OPUks.
In order to solve the above problems, the following methods are adopted in the prior art.
Firstly, under a situation of OPUk-2v (2 virtually-concatenated OPUks), every 2 bytes in the residual bytes of the OPUk-2v channel overhead are distributed to one Cbyte, and the errors are filtered by adopting the majority vote manner. Furthermore, a byte space of 3808×4×2 for the OPUk-2v can be completely indicated. One of the possible distribution solutions is as shown in the OPUk-2v frame structure shown in FIG. 2.
Next, under a situation of OPUk-Xv (X≧3) (more than 3 virtually-concatenated OPUks), 3 bytes are distributed to one Cbyte, and the Cbyte occupying 3 bytes can indicate a maximum byte space of 3808×4×256 for the OPUk-Xv (that is, at most X=256 virtually-concatenated OPUk frames can be supported). Furthermore, at least 3 virtually-concatenated OPUk frames exist, so that 3 Cbyte spaces are distributed in the OPUk-Xv OH for performing the majority vote. The detailed frame structure may be obtained with reference to an OPUk-3v frame structure shown in FIG. 3.
During the research and practicing process in the prior art, it was found that the following problems exist in the prior art.
First, as the byte number of the byte space of the Cbyte is increasing, if an error occurs to any one bit among 16 bits when being transported, the whole Cbyte value becomes incorrect. By simply transporting 3 same Cbytes in each frame and adopting the majority vote manner, the reliability and the error tolerance are not desirable. Furthermore, 3 same Cbytes are transported in each frame, so that excessive overhead bytes are occupied, which is not helpful for further standardized expansion.
Next, no prior art CBR mapping frame structure with a uniform format is provided for the OPUk-Xv, which is a disadvantage for achieving the uniformity of the CBR mapping frame structure. In the above prior art CBR mapping method, the client signal byte number Cn transported in the OTN frame period has become an important factor that influences the reliability.